The invention relates to automatic shut-down systems for nuclear reactors, and more particularly to an automatic safety rod for such shutdown systems which includes a pressure regulating device to prevent undesired shutdown due to a decrease of coolant flow.
The use of control systems to regulate the reactivity of a nuclear reactor by varying the location of control (neutron absorber) elements with respect to the reactor core is well known. With a view toward the possibility of an emergency condition arising, as by an unexpected drop in coolant flow or rise in reactivity, such control systems include arrangements for "scramming" the control rods; i.e., for rapid insert of the absorber elements into the core to quickly shut down the reactor.
With the advent of the liquid metal fast breeder reactor (LMFBR), a need for faster, less complex, more reliable control rod scram or shutdown systems has become apparent, whereby the reactivity of the reactor can be quickly shut down.
More recent efforts have been directed to the desirability of utilizing secondary or alternate control systems of the self-actuating type which would make a liquid metal fast breeder reactor (LMFBR) inherently safe. Such alternate or self-actuating systems provide control without reliance on the primary reactor control system or plant operators, while being capable of actuation by the plant operators. These efforts have resulted in systems which sense the reactor flow rate and actuate when the flow drops below a predetermined level, or sense the coolant temperature and actuate when it exceeds a certain level, or measure the neutron flux or reactivity level of the reactor and actuate when the neutron flux or reactivity exceeds a specified level.
Also, recent efforts have been directed to utilizing a self activated shutdown system (SASS) in a gas-cooled fast reactor (GCFR), wherein the lower core pressure drop imposes a more severe limitation on the operation capability at partial flow. The SASS for the GCFR, like the LMFBR, usually requires a built-in energy absorbing dashpot to prevent damage to the falling control rod.
One general type of shutdown system utilizes control rods held in the uppermost, or cocked, position hydrostatically by the differential fluid pressure across the reactor core. If this pressure decreases below a design lower limit, the control rod will fall by gravity into the reactor core and stop the chain reaction. Resetting of the rods is accomplished only by deliberately lowering a lifting grapple to engage the control rod and return it to the uppermost or cocked position while the coolant is pumped through the core. The grapple is then disengaged and retracted.
These prior known hydrostatic or coolant pressure holdup type control systems are exemplified by U.S. Pat. Nos. 3,115,453 issued Dec. 24, 1963 to J. A. Paget et al; 3,940,310 issued Feb. 24, 1976 to L. Irion et al; and 3,980,519 issued Sept. 14, 1976 to W. E. Taft.
An inherent disadvantage of the above-described hydrostatic system is that either it prevents deliberate operation of the reactor at reduced flow, or it takes too long after pump failure, for example, for the flow to decrease from its rated or hold value to the release value. Efforts for overcoming this disadvantage by the use of bellows and gas chambers are exemplified by U.S. Pat. No. 4,167,443 issued Sept. 11, 1979 to R. C. Noyes et al and by report EPRI-NP-846 entitled "Self-Activated Shutdown System for a Commercial Size LMFBR", August 1978, by Combustion Engineering, Inc., Windsor, Conn.
While these prior systems have been effective to a certain extent, a need exists for a hydrostatic type automatic control system which allows for a deliberate reduced coolant flow operation without "scramming" the system.
Therefore, an object of this invention is to provide an automatic safety rod for nuclear reactors.
Another object of the invention is to provide a pressure actuated control rod arrangement for a self activated shutdown system.
Another object of the invention is to provide a control rod having an actuating mechanism which compensates for certain reduced coolant flow operations of the reactor.
Another object of the invention is to provide an automatic control rod for a nuclear reactor which utilizes a pressure regulating device whereby the reactor can operate under selected reduced coolant flow conditions without activation of the control rod to scram the reactor.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.